Straddling and Overriding AV Valves
This defect is characterized by a wide spectrum of deformities, primarily occurring between the AV valve papillary muscle apparatus and their attachments to the interventricular septum.
On one end of the spectrum is a double inlet right ventricle, and on the other end of the spectrum is a double inlet left ventricle. At both extreme ends of the spectrum, whether it is double inlet left right ventricle or double inlet left ventricle is essentially a univentricular heart with either a very hypoplastic right ventricle, or a very hypoplastic left ventricle.
As a result, this defect can resemble both an endocardial cushion type of defect (AV Canal) or a univentricular heart. Inherent in this defect are two coherent AV valves that are attached concordantly to each atrium with a normally attached atrial septum at the atrioventricular junction (crux of the heart). It is the deviant attachment of the papillary muscles of one of the AV valves to the interventricular septum that characterizes this defect.
For example, if one set of papillary muscles from the mitral valve are attached to the right ventricular side of the ventricular septum, this causes the ventricular septum to deviate leftward, in turn causing the mitral valve to override and straddle the ventricular septum. This causes both atria to drain towards the right side.
There are varying degrees of straddling, from mild to severe. In milder cases, part of the atrial blood will drain into the left ventricle and the rest will drain into the right ventricle. As the straddling becomes more severe, the left ventricle decreases in size as more blood drains to the right side (double inlet right ventricle). In its most severe form, the LV will be severely hypoplastic, almost all of the blood will drain rightward, and this will resemble a univentricular right heart.
Vice versa. If the papillary muscles of the TV are attached to be left ventricular side of the ventricular septum, this moves the ventricular septum to the right, and the same process outlined above will happen to the right side of the heart, resulting in double inlet left ventricle, or in severe cases, univentricular left heart.
Needless to say, there is usually a large VSD with relatively complete mixing of ventricular blood. As a result of the intact AV junction, the semi-lunar valves and great vessels remain patent, and mixed blood will flow primarily from the dominant ventricle through both great vessels.
There is one other defect to consider, and that is a common valve, or a solitary AV valve in which the papillary muscles connect to both ventricles and override a common ventricular septum. In this case, the common valve will either be a right or left AV valve, and there are varying degrees of dominance of one ventricle over the other.
Palliative procedures may include a pulmonary band, redirection of circulation at either the atrial or ventricular levels, systemic to pulmonary shunts or attempts to repair regurgitant valves.
Surgical repairs always depend upon the severity of the lesion and a bi-ventricular repair is the preferred option. However in severe cases, a Fontan repair is usually performed.
Ken Heiden RDCS